Television system



Oct. 15, 1940. c, FAUDELL AL 2,218,067

TELEVISION SYSTEM Filed Feb. 4, 1939 Fig.1

I NV EN TOR 5 C.L.FAUDELL, R. ESPENCER N .J. RJAMES ATTORNEY.

Patented Oct. 15, 1940 UNITED STATES PATENT OFFICE 'ranavrsron srs'rarr of Great Britain Application February 4, 1939, Serial No. 254,574 In Great Britain February 4, 1938 7 Claims.

The present invention relates to systems for converting impulses and has particular but not exclusive reference to television systems where it is desired to separate pulses of the same ampli- .-tude but of different duration. In certain known television systems synchronisation is effected by using synchronising pulses which are transmitted in the opposite sense to the picture signals, so that the synchronising 10 pulses can be separated from the picture signals by amplitude selection. In such systems it is usual to make the frame synchronising pulses of longer duration than the line synchronising pulses but of substantially the same amplitude and various methods have been proposed forthe separationof the frame and line pulses.

Since the frequency of the frame synchronising pulses is much lower than the frequency of the line synchronising pulses it has been proposed toseparate the line pulses from the frame pulses by passing the pulses through a low'pass filter,

the resulting pulses being applied either to an amplitude filter or directly to a sawtooth oscillation generator, such, for example, as a blocking oscillator which requires a pulse of greater than a critical amplitude to trigger it.

The disadvantages of such a method have been discussed in British patent specification No.

455,375 and an alternative method of pulse sepdiffering amplitudes, which pulses were subsequently passed to the sawtooth oscillation generator.

provide an improved method and apparatus which facilitates the separating of a set of impulses from another set of impulses of a different duration.

I According to the present invention a. method is provided of converting impulses of substantially the same amplitude but of different durae tions into impulses of different amplitudes which method comprises applying the impulses of the same amplitude to a converting circuit from which derived impulses are obtained, said converting circuit serving to convert the applied impulses into two portions, the first portions being of the same sign as the applied impulses and the second portion being of the opposite sign to the applied impulses, the applied impulses of shorter duration producing impulses of small amplitude ditional step of separating aration was suggested therein, whereby the pulses It is the object of the present invention to in the second portion and the applied impulses of longer duration producing impulses of relatively larger amplitude inthe second portion.

According to a further feature of the invention, the derived impulses are applied in such a way to the applied impulses that the first por- 5 tion of-the derived impulses is effectively can-, celled. V A The present invention also comprises the adthe converted' pulses by amplitude selection.

Where the method according to the invention is employed in a television receiving system, the

converted pulses may be-applied to a. generator of frame scanning oscillations which is so, ar-

ranged as to be afiected'substantially only by the 15 impulses of relatively larger amplitude.

Whereas in the prior specification No. 455,375 separation is afiected by an integration process,- it will be apparent that the converting circuit employed in the present invention functions in a 20 manner which is analogous to difierentiation.

The invention also comprises an apparatus for carrying out the methods-set forth above.

In order that the said invention may be clearly understood and readily carried into effect, it will 2 now be more fully described with reference to the accompanying drawing, in which:-

Figures 1, 2 and 4 are explanatory diagrams,

and

Fig'uTes 3 and -5 are circuit arrangements for carrying out the methods according to the in-. vention.

" Referring to Figure 1 which shows a portion of the wave-form of the synchronising pulses-of.

a well-known system of television, the pulses marked a are the line synchronising pulses and those marked b are the frame synchronising pulses. It will be appreciated that the picture signals (not shown) will be superimposed on the waveform in the opposite sense to the direction of 40 the synchronising impulses. In this system the duration of a pulse b is four times as long as the duration of a. pulse a, the amplitudes being the same.

pulses being applied inthe positive sense. The

Referring to Figure 3, which shows a, circuit 5 screen grid 8 of the valve is connected to a positive source of potential through resistance l5, whilst the anode is also connected through a resistance l6 to a source of positive potential. As is well known synchronising pulses in the negative sense and free from picture signals are obtainable from the screen grid 8 and the anode 9 of the valve due to the self-biasing action of the grid circuit, the waveform being of the form shown in Figure 1. If necessary the potentials at the screen grid 8 may be conveyed to the line scanning oscillator for the purpose of synchronising it.

To the anode 9 is connected a condenser l1 and a resistance 3 in series, the end of resistance 18 being connected to the negative source of potential 20. By a suitable choice of the time constant of the converting circuit comprising the resistance and condenser it is possible partly to differentiate the pulses obtained on the anode 9 thus obtaining pulses at the junction H! of the resistance l8 and condenser l1, which pulses are of different amplitudes and are in a positive sense and are superimposed on the pulses in the negative sense. The positive pulses obtained across the resistance l8 thus comrpise small pulses due to the line pulses and larger pulses due to the frame pulses, the waveform being similar to that shown in Figure 2.

It will be seen from Figure 2 that at the end of a line pulse a pulse 0 of small amplitude is produced in the opposite sense to the line pulse, whilst at the end of a frame pulse a pulse 11 of greater amplitude-than pulse 0 is also. produced in the opposite sense to the frame pulse. It is possible to separate these pulses c and d by amplitude separation or it is possible to apply the differentiated waveform directly to the synchronising electrode of the sawtooth oscillation generator. The time constant of the differentiating circuit should be of the order of or greater than the duration of a line pulse and less than the duration of a frame pulse. Although in Figure 3 a particular type of amplitude separator is described, it will be obvious that any other method of separa tion of picture signals from synchronising pulses is applicable. It is also to be understood that an inductance may be used for the purpose of differentiating, or other means may be provided for the purpose of reducing the lower frequency components of the synchronising pulses.

A further featureof the invention consists 'in mixing the original synchronising pulse waveform as in Figure l in opposite phase with the differentiated waveform of Figure 2. Bythis means a waveform of the type shown in Figure 4 is obtained. It will be seen that the line synchronising pulses produce very small pulses as shown at c, whilst the frame synchronising pulses produce much larger pulses I. These large pulses may be used directly to control the frame scanning generator or they may be separated from the small pulses by amplitude separation before utilization. If desired the frame frequency generator may be triggered during theoccurrence of any of the pulses of larger duration. For example it may be arranged that the frame frequency generator is triggered before a half-line period has elapsed from the beginning of a frame pulse.

A circuit arrangement in which the further feature of the invention may be incorporated is shown in Figure 5. In Figure 5 the valve 2| is used for the purpose of separating the synchronising pulses from the picture signals. This circuit also'employs the'self-biasing method of separation, the grid condenser 25 being connected to the input terminal 30 and the other input terminal 3| being connected to the cathode 24 of the valve 2| and to the grid leak resistance 26. Between the cathode 24 and a negative source of potential 33 is connected a resistance 28. The anode 23 of the valve is connected through resistance2l'to a positive source of potential and a condenser 29 is connected between the anode 23 and the cathode 24 and terminal 32. The output is taken from terminals 32 and 33.

In operation, composite television signals are applied between terminals 30 and 3| with the synchronising pulses in a positive sense. Due to the limiting action of the grid circuit the cathode of the valve tends to follow the synchronising pulses. Simultaneously, however, the differentiated pulses passed through condenser 29 tend to oppose the cathode variation with the result that pulses of the waveform shown in Figure 4 appear across the output terminals 32 and 33, and these may be applied directly to the frame scanning generator or to an amplitude separator or other device. Of course, systems of picture signal separation other than that shown are equally applicable to this method of pulse selection.

Alternative methods of mixing the original and differentiated pulses are possible. For example, a signal as shown in Figure 1, may be applied to one control grid of a hexode valve and a differentiated signal, as in Figure 2, may be applied in the opposite sense to the other control grid of the hexode, the output being taken from the anode circuit. In this case a triode-hexode type of valve can be conveniently used, the triode section forming the picture signal separator. Also circuits using two triodes, for example, may be employed for the purpose of additively mixing the two types of waveform.

' ing anode, cathode and at least onecontrol electrode, a time constant circuit connected in the control electrode-cathode circuit of said tube, means for impressing said impulses of substantially the same amplitude but differing duration onto said timeconstant circuit, a circuit for producing a wave form of a differential of the wave form applied to the time -consta nt circuit connected in the anode-cathode circuit of said tube, and terminal means for taking from a portion of said differentiating circuit impulses of differing amplitudes.

2. Apparatus in accordance with claim 1 wherein said differentiating circuit comprises a condenser and a resistive memben serially connected, and the terminals for withdrawing the impulses of differing amplitudes are connected across said resistance element.

3. Apparatus in accordance with claim 1 wherein said thermionic tube comprises a multigrid tube.

4. Apparatus for converting impulses of substantially the same amplitude but of differing durations into impulses of diii'ering amplitudes comprising a thermionic 'vacuum tube having anode, cathode and at least one control electrode,

a. timeconstant'circuit connected in the control,

electrode-cathodecircuit oi said tube, means for impressing said impulses of diflering duration onto said time constant circuit, a circuit for producing a wave form bearing a diiferential relationship to the wave form applied to the'time constant circuit, said latter circuit being connected in the anode-cathode circuit 01' said thermionic tube, means for combining the impulses impressed on the control electrode oisaid tube with the differential wave form developed, and terminal means for withdrawing the wave form resulting from the combining of the diflerential wave form with the original impulse wave form.

5. The method of converting electrical impulses having substantially the same amplitude but ditfering durations into impulses having diflering amplitudes which comprises the steps of developing a wave form bearing a difierentlal relationship to the original impulses, said wave form having a portion thereof corresponding to the original wave form and a second portion thereof diifering from the original wave form, and combining said developed wave form with said original wave form in such a manner that the similar portions of each of the wave forms is cancelled out.

i 6. Apparatus for converting electrical impulses having substantially the same amplitude but'of differing duration into impulses having differing amplitudes comprising a thermionic vacuum tube having anode, cathode and at least one control electrode, means for impressing said impulses to be converted onto the control electrode-cathode circuit of said tube, means for energizing the anode of said tube, resistance means connected serially with said anode energizing means and with said anode, a condenser substantially shunted across the anode and at least one side of the cathode of said tube, a second resistance means connected serially with the negative side of said anode energizing means and the cathode of said tube, and terminal means for withdrawing the wave form developed across said second resistive means.-

'7. Apparatus in accordance with claim '6 wherein there is provided in addition, a time constant circuit connected in the control electrode-cathode circuit of said tube, 

